In the related art, a film forming device for forming a film on a surface of a conductive workpiece such as steel material by using plasma has been frequently suggested.
For example, the background art discloses a technology of forming a diamond-like carbon (hereinafter, referred to as ‘DLC’) film on the surface of the workpiece.
According to the technology disclosed in the background art, a plasma generating device is configured to supply microwaves towards a workpiece in a treatment chamber through a quartz window, which is a microwave transmitting window, thereby generating plasma in a peripheral area of an inner surface of the quartz window. The workpiece has a rod shape, for example, and is arranged to protrude from the inner surface of the quartz window into the treatment chamber, and a sheath layer is generated on the workpiece at the peripheral part of the inner surface of the quartz window covered by the generated plasma. Subsequently, the plasma generating device applies a negative bias voltage to the workpiece while supplying of the microwaves.
As a result, the thickness of the sheath layer formed on a surface of the workpiece expands along the surface of the workpiece. At the same time, the supplied microwaves propagate along the expanded sheath layer, as surface waves with a high energy density. At this time, the plasma and the sheath layer are also generated on the surface of the workpiece away from the periphery of the inner surface of the quartz window by the surface waves with a high energy density. The thickness of the newly generated sheath layer is also expanded by the negative bias voltage, and the microwaves further propagate along the expanded sheath layer, as the surface waves with a high energy density. Thereby, the plasma extends towards the part of the workpiece away from the periphery of the quartz window, i.e., towards the other end of the workpiece, which protrudes into the treatment chamber from one end of the workpiece facing the quartz window. As a result, a source gas is plasma-excited by the surface waves and becomes high density plasma, so that a DLC film is formed on the entire surface of the workpiece.